Why can't I find my desired attribute in the Category Page Identifier dropdown?

In order for an attribute to appear in the Category Page Identifier dropdown, it must be (1) included in your attributesForFaceting and (2) be made searchable. 
 
You can make this update for your attribute as need by following this guide and substituting your  desired attribute(s) for the categoryPageId attribute used in the guide's example.
 
Once you make this update, you'll have access to these attributes as Category Page Identifiers.